1. Field of the Invention
The present invention relates to a modem apparatus that demodulates reception symbols and identifies the type of a control signal, an image communication apparatus equipped with this modem apparatus and communication control method.
2. Description of the Related Art
The Recommendation V.34 on a modem specified by the ITU-T describes a half-duplex system for facsimile communications that executes a protocol for exchanging modem and facsimile control signals according to a full-duplex system using a control channel in addition to a primary channel.
When transmitting an image signal according to the Recommendation V.34, the image communication apparatus on the calling side (hereinafter referred to as “calling terminal”) must transmit either a PPh signal or Sh signal at the beginning of a control channel. The PPh signal is used when the communication speed of the next primary channel is negotiated under the exchange protocol of the control signal. On the other hand, the Sh signal is used when the communication speed of the next primary channel need not be negotiated because the communication speed of the next primary channel is already determined.
When an image signal according to the Recommendation V.34 is received, the image communication apparatus on the answering side (hereinafter referred to as “answering terminal”) must identify whether the signal received at the beginning of the control channel is a PPh signal or Sh signal. When a PPh signal is received, the answering terminal sends back a PPh signal at the beginning of the next control channel. On the other hand, when an Sh signal is received, the answering terminal sends back an Sh signal or PPh signal at the beginning of the next control channel. Therefore, the calling terminal also needs to identify whether the answering terminal sends an Sh signal or PPh signal at the beginning of the control channel.
FIG. 1A shows a case where the calling terminal (TX) is sending an Sh signal at the beginning of the control channel and the answering terminal (RX) is sending back an Sh signal in response to this. On the other hand, FIG. 1B shows a case where the calling terminal (TX) is sending a PPh signal at the beginning of the control channel and the answering terminal (RX) is sending back a PPh signal in response to this.
The Recommendation V.34 uses a quadrature amplitude modulation system (e.g., QAM modulation system) as one of modulation systems of control signals (PPh signal, Sh signal, etc.) exchanged on a control channel. FIG. 2A shows a signal space diagram (IQ plane coordinates) in QAM modulation. The QAM modulation system is a system in which phase modulation is performed at a multiple (1 to 8) of 45°, which is a unit angle, in the range of 0° to 360°. That is, symbols that make up the control signal are modulated with a value obtained by multiplying one modulation unit (45°) by one of integers “0”, “1”, “2”, “3”, “4”, “5”, “6” or “7”.
The Recommendation V.34 defines modulation patterns for all control signals exchanged on a control channel. In the case of a PPh signal, 8 known symbols are phase-modulated in the order of 11311535 and this is repeated 4 times. In the case of an Sh signal, 8 known symbols are phase-modulated in the order of 13131313 and this is repeated 3 times and then 8 known symbols of an Sh bar signal are phase-modulated in the order of 57575757 and this is sent only once.
The modem apparatus obtains the modulated phase from coordinates of reception symbols in a signal space diagram when reception symbols are QAM-demodulated. Then, when the modulation pattern with which reception symbols are modulated matches the modulation pattern of the PPh signal, the modem apparatus detects that the PPh signal has been received. When the modulation pattern with which reception symbols are modulated matches the modulation pattern of the Sh signal, the modem apparatus detects that the Sh signal has been received.
However, it is sometimes difficult to make a distinction when there is a shift of oscillation frequency between the calling side and answering side or when coordinates of two consecutive symbols come closer to each other or a phase rotation occurs due to influences of the channel characteristic. For example, it is known that when “1” and “3” are repeated as in the case of quadrature amplitude modulation on an Sh signal, the reception state becomes such that the coordinates of a reception symbol corresponding to the modulation of “1” come closer to the coordinates of a reception symbol corresponding to the modulation of “3” as shown in FIG. 2B. In such a reception state, it is difficult to distinguish whether the reception symbol is “1” or “3”. FIG. 2C shows a state in which the phase of a reception symbol corresponding to the modulation of “3” further rotates toward the “1” side, and in this way the reception symbols corresponding to “1” and “3” enter into the same quadrant. In such a reception state, it is now difficult to achieve accurate demodulation unless the rotated phase is compensated.
Thus, the conventional way of detecting coordinates on a signal space diagram of reception symbols, determining a modulation pattern of consecutive reception symbols and thereby identifying a control signal (Sh signal, etc.) has a problem that it is difficult to identify a control signal, Sh signal in particular, when coordinates of two consecutive symbols come closer to each other or a phase rotation occurs beyond a quadrant boundary. This means that failing to detect an Sh signal in the control channel will result in an AC sequence, thereby producing considerable time losses.